The paradoxical evolution of sensor SNR over time

We have seen that SNR has decreased by 1dB in the last five years. However, it is
probably clear to all photographers that overall image quality has
significantly increased during the same period. Why is this? Simply put, the
number of pixels has doubled during the same time period, which means that when
viewing at full screen or when producing a print, twice as many pixels will be
averaged to achieve the screen or print resolution, resulting in a greatly
improved SNR.

Let’s
normalize the SNR for a 20x30cm print at 300dpi, which is roughly equivalent to
an 8Mpix photo. The following graph shows the normalized SNR as a function of
the camera release date. Cameras with a full field sensor format (24x26mm) are
displayed in blue, and cameras with an APS-C sensor format (around 15x22.5mm)
are displayed in green. (Intermediate formats such as 19x29mm—e.g., that of
the Canon 1DMIII—have not been plotted.)

SNR at ISO200 (db), normalized for a 20x30cm print

We
see that the full-frame sensors (blue dots) have a clear advantage over the
APS-C sensors (green dots). This is because full-frame sensors are
collecting twice as much light as APS sensors. It is easy to see that the
average difference of SNR is about 3dB (a one-stop gain),4 which is
exactly what is expected when doubling the amount of light.

We
also see a greater improvement over time for full-frame sensors than for
APS-Cs. In five years, full-frame sensors have gained 5dB, which is a gain of
nearly two f-stops! For the same period (roughly from 2003 to 2005), APS
sensors have gained only 2 dB. However, there has been only limited progress in
improving SNR over the past three years or so.

Based
on most photographers’ experience, we would probably expect much more
improvement on the APS side — after all, the quality of a 2008-model camera at
ISO400 is much greater than the quality of a 2003-model camera at ISO200! Why
isn't this the case?

The
answer lies in the fact that we are not looking directly at RAW images, but
rather at RGB images after RAW conversion. Sensors are not the only element of
image quality that have advanced over these past few years. RAW converter
algorithms, whether embedded in the camera or as software in a PC (e.g., Adobe
Photoshop, DxO Optics Pro, etc.) have also made large advances. For example,
over the past two years, DxO Optics Pro RAW conversion (with its denoising
algorithm) showed a gain of +4dB in the SNR, a much larger improvement than
that achieved for sensor technology.

These
advances in RAW conversion algorithms will continue, underscoring the advantages of
shooting in RAW, as RAW images can always be reprocessed with every new release
of a RAW converter. Of course, however advanced RAW converters become, the best
final image will still rely on shooting the best RAW image possible.

Notes

4This
also means that for a 20x30cm print, the image quality of an APS-C camera at
ISO200 will be on average equal to the image quality of a full-frame camera at
ISO400.